The production of gastric acid is dependent of the function of an enzyme, H+ K+ ATPase, that is capable of pumping protons into a secretory canaliculus against a 6-log concentration gradient. The OH- that is generated by this pump is converted to HCO3- by the action of a second enzyme, carbonic anhydrase II (CA II). previous studies have demonstrated that the gastric mucosal expression of the genes encoding these two parietal cell enzymes undergoes major changes during development. Steady state levels of H+ K+ ATPase mRNA are low at birth and increase rapidly at 3 weeks of age. These changes are accompanied by parallel developmental changes in acid secretory capacity. In contrast, CA II mRNA levels are high in the newborn but rapidly decline over the next 3 weeks of life. This reciprocal relationship between CA II and H+ K+ ATPase during development contrast with the parallel regulation of the genes encoding the two enzymes in parietal cells by acid secretagogues. An indication as to the basis of this disparity was provided by previous studies localizing the expression of CA II by in situ hybridization to all of the cells in the gastric mucosa of the newborn but primarily to cells at the glandular base and surface epithelium in mature animals. These data served to underscore the potential importance of CA II in maintaining cellular pH under circumstances other than during acid secretion: during cell division such as in developing animals or in the process of renewal of glandular elements and during HCO3-secretion by surface epithelial cells in the stomach as a first line of mucosal defense against the acid environment of the gastric lumen. Thus, an understanding of the regulation of CA II gene expression would shed light on a multitude of important functions of the enzyme in gastric mucosa. Accordingly, the objective of this proposal is to examine at a genetic level, the mechanism by which the expression of CA II is regulated . Toward this goal, the following specific aims are proposed: Characterized the cis-regulatory regions of the CA II gene by linking various portions of the gene's promoter/enhancer region (with or without mutation and/or deletions) to a chloramphenicol acetyl transferase (CAT) expression system; identify and characterize specific nuclear proteins which may serve as transfactors to regulate promoter/enhancer/silencer activity in the cis-regulatory regions; determine if the CA II gene is transcriptionally regulated during development and if cis-trans interactions thus identified by gel retardation assays regulated transcription in vitro; clone the cDNA(s) encoding transfactors that regulate CA II gene expression. We hope that the work included in this proposal will shed light on the important mechanism by which the CA II gene is regulated during development and believe that the information obtained may have broad significance with respect to the entire process of growth and development.